Axle driving system

ABSTRACT

An axle driving system which houses in a housing thereof a hydrostatic transmission, axles, and a driving gear train for connecting output means of the hydraulic transmission and axles, so as to transmit power from a driving source to the hydrostatic transmission and to change the speed, thereby driving the axles. A first chamber therein contains the hydrostatic transmission and a second chamber therein contains the driving gear train. Both the first and second chambers are independent of each other so as to prevent a foreign object, such as iron powder produced in the driving gear train, from entering the hydrostatic transmission. The system includes an L-like-shaped center section on which the hydrostatic transmission is offset such that an imaginary plane which includes a motor mounting surface passes in proximity to the axis of a pump shaft. The pump shaft is disposed perpendicular to the axles. The motor shaft is disposed in parallel thereto. A hydraulic pump is positioned between the hydraulic motor and the axles, so that the housing for the hydrostatic transmission, axles and driving gear train, is smaller in width to thereby make the system more compact.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an axle driving system in whicha hydrostatic transmission (hereinafter referred to as an “HST”), axlesand a power transmitting mechanism are integrally provided in a housing,and more particularly to an axle driving system in which the width ofthe portion of the housing which houses the HST and power transmittingmechanism is smaller than in conventional systems.

[0003] 2. Related Art

[0004] A conventional axle driving system houses the HST, axles and adriving gear train for interlocking the HST with the axles in a commonhousing. The HST is constructed so that a hydraulic pump is disposed ona horizontal portion of a center section which is L-like-shaped and ahydraulic motor is disposed on the vertical portion of the same. Thehydraulic motor is position to one side of the axle. The hydraulic pumpand hydraulic motor are fluidly connected to each other by a closedfluid circuit formed in the center section. The hydraulic pump is drivenby a prime mover provided on the vehicle so as to drive the hydraulicmotor and then the axles through a driving gear train. Such aconstruction is disclosed, for example, in U.S. Pat. Nos. 5,163,293 and5,335,496.

[0005] The hydraulic pump and hydraulic motor in the conventionaltechnique, are disposed side-by-side and to one side of the axles. Assuch, the width of the HST is larger which results in the lateral widthof the common housing for both the pump and motor also being larger.Furthermore, an output shaft of the hydraulic motor extends to one sideof the vehicle to transmit power therefrom to a differential gear unitthrough gears of a driving gear train, so as to drive the axles. Anunused space is formed at a side of the gear train and between the HSTpump and the axles.

[0006] Further, when the HST and the driving gear train for driving theaxles by the output shaft of the HST are housed in a common housing, aforeign object, such as iron powder produced by the driving gear train,may enter into the HST. This can adversely affect operation of the HSTor various parts thereof.

SUMMARY OF THE INVENTION

[0007] The axle driving system of the present invention is constructedso that the HST center section is formed in such a manner that theextended phantom plane of the motor mounting surface of the centersection passes in the vicinity of the axis of the pump shaft of thehydraulic pump. The pump shaft extends substantially perpendicular tothe axles. The motor shaft of the hydraulic motor extends substantiallyin parallel thereto. The hydraulic pump is disposed between thehydraulic motor and the axles. Hence, the width of the housing is madesmaller so as to be compact in size. The axle driving system, which issmaller in lateral width, is provided with a wide swinging space for therunning wheels of the vehicle and is extremely effective for a vehiclehaving freely steerable wheels mounted thereon.

[0008] Further, the present invention divides the housing into twoseparate chambers for housing the HST and for housing a driving geartrain and axles. A partition for dividing the two chambers is providedwith an oil filter so that both chambers can be filled with common oil.This improves the durability of the HST and reduces the manufacturingcost.

[0009] The above and other related objects and features of the inventionwill be apparent from a reading of the following description of thepreferred embodiments including the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE FIGURES

[0010]FIG. 1 is a partial cross-sectional plan view of a firstembodiment of an axle driving system of the present invention, fromwhich an upper half housing is removed;

[0011]FIG. 2 is a cross-sectional view looking in the direction of thearrows 2-2 in FIG. 1;

[0012]FIG. 3 is a cross-sectional view looking in the direction of thearrows 3-3 in FIG. 1;

[0013]FIG. 4 is a cross-sectional view looking in the direction of thearrows 4-4 in FIG. 1;

[0014]FIG. 5 is a cross-sectional view looking in the direction of thearrows 5-5 in FIG. 1;

[0015]FIG. 6 is a cross-sectional view looking in the direction of thearrows 6-6 in FIG. 1;

[0016]FIG. 7 is a top plan view of a center section of the presentinvention;

[0017]FIG. 8 is a side elevational view of the same;

[0018]FIG. 9 is a bottom plan view of the same;

[0019]FIG. 10 is a cross-sectional view looking in the direction of thearrows 10-10 in FIG. 7;

[0020]FIG. 11 is a cross-sectional view looking in the direction of thearrows 11-11 in FIG. 8;

[0021]FIG. 12 is a cross-sectional view looking in the direction of thearrows 12-12 in FIG. 8;

[0022]FIG. 13 is a cross-sectional view looking in the direction of thearrows 13-13 in FIG. 7;

[0023]FIG. 14 is a cross-sectional view looking in the direction of thearrows 14-14 in FIG. 7;

[0024]FIG. 15 is a cross-sectional rear view of a portion of the presentinvention surrounding a brake operating shaft;

[0025]FIG. 16 is a cross-sectional view looking in the direction of thearrows 16-16 in FIG. 15;

[0026]FIG. 17 is a cross-sectional view looking in the direction of thearrows 17-17 in FIG. 15;

[0027]FIG. 18 is a perspective view of the brake operating shaft and abiasing member of the present invention;

[0028]FIG. 19 is a plan view of a second embodiment of the axle drivingsystem of the present invention from which an upper half housing isremoved;

[0029]FIG. 20 is a cross-sectional view looking in the direction of thearrows 20-20 in FIG. 19;

[0030]FIG. 21 is a sectional view looking in the direction of the arrows21-21 in FIG. 19;

[0031]FIG. 22 is a side view of an alternative embodiment of the centersection of the-present invention;

[0032]FIG. 23 is cross-sectional view looking in the direction of thearrows 23-23 in FIG. 22;

[0033]FIG. 24 is a cross-sectional view looking in the direction of thearrows 24-24 in FIG. 22; and

[0034]FIG. 25 is a cross-sectional view looking in the direction of thearrows 25-25 in FIG. 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Explanation will now be given on the entire construction of anaxle driving system according to the present invention in which thehousing thereof comprises an upper half housing 1 and a lower halfhousing 2 which are joined together along a horizontal, flat peripheraljoint surface of each half housing. Along the joint surface of the upperand lower half housings is provided bearings for a motor shaft 4 and acounter shaft 26. Axles 7 are disposed in parallel to the joint surfaceof the housing. The bearings-for axles 7 are shifted upwardly from thejoint surface and are disposed in upper half housing 1 so as torotatably support axles 7. Axles 7 are differentially coupled with adifferential gear unit 23. Each axle 7 projects outwardly from one endof left and right side walls of the housing, respectively.

[0036] The interior of the housing is divided by an inner wall 8 into afirst chamber R1 for housing therein an HST and a second chamber R2 forhousing therein (1) a driving gear train comprising a plurality of gearsfor transmitting power from motor shaft 4 to differential gear unit 23,(2) differential gear unit 23, and (3) axles 7. Inner wall 8 comprises alongitudinal portion which is in parallel to axles 7 and a perpendicularportion which extends at a right angle to the longitudinal portion ofinner wall 8. Both portions of inner wall 8 are continuously provided sothat first chamber R1 is disposed adjacent to second chamber R2. Innerwall 8 also comprises a vertical wall portion which extends downwardlyfrom the interior of upper half housing 1 toward the joint surface ofthe housing and rising from the interior of second half housing 2 towardthe same. The end surfaces of both the vertical wall portions of innerwall 8 abut against each other when both upper and lower half housings 1and 2 are joined, thereby forming two divided, independent chamberswithin the housing.

[0037] The first and second chambers R1 and R2 are filled withlubricating oil which is used in common therewith to form an oil sump.As shown in FIG. 6, an oiling lid 6 is provided on an upper wall ofupper half housing 1 above differential gear unit 23 so as to enableoperating oil to be supplied through lid 6. As shown in FIG. 5, an oilflow-through port 75 is mounted on a wall surface of upper half housing1 constituting first chamber R1, so that first chamber R1 and anexternal reservoir tank 10 fluidly communicate with each other through apiping 9 made of a rubber hose or the like so as to enable operating oilin the oil sump to be maintained at a predetermined amount. The amountcan be adjusted by flowing an incremental volume of oil into reservoirtank 10 when the temperature of the oil rises when the HST is driven.

[0038] An oil filter 18 is disposed on inner wall 8 which partitionsfirst chamber R1 from second chamber R2. In a first embodiment, as shownin FIGS. 1 and 5, oil filter 18 is disposed at the joint surfaces of thevertical portions of inner wall 8 to house therein the HST and rightside axle 7, thereby enabling oil to flow through oil filter 18 betweenfirst chamber R1 and second chamber R2. Accordingly, oil provided in thehousing can be used in common as operating oil for the HST and aslubricating oil for the gears and bearings. Also, when oil flows fromsecond chamber R2 into first chamber R1, harmful foreign objects such asiron powder, flowing into the HST is filtered by oil filter 18.

[0039] First chamber R1 is disposed in front of axles 7 and to the sideof the geared transmission for transmitting power from motor shaft 4 todifferential gear unit 23, provided in the housing. A center section 5of the HST is mounted in first chamber R1 and is separate therefrom.Center section 5 is disposed in a manner such that its longitudinaldirection is substantially perpendicular to axles 7. The front portionforms a vertical surface 91 on which a motor mounting surface 41 isformed on which a hydraulic motor is disposed. The rear portion forms ahorizontal surface 90 on which a pump mounting surface 40 is formed onwhich a hydraulic pump is disposed. Accordingly, the hydraulic pump isdisposed between the hydraulic motor and axles 7. A pump shaft 3 issupported vertically in the center of pump mounting surface 40 and ispositioned between the hydraulic motor and axles 7.

[0040] The axial piston type hydraulic pump of the present inventionincludes a cylinder block 16 which is rotatably, slidably disposed onpump mounting surface 40 of center section 5. Pistons 12 are fitted intoa plurality of cylinder bores and move in reciprocation through biasingsprings. A movable swash plate 11 having a thrust bearing 11 a abutsagainst the heads of pistons 12. At the center of movable swash plate 11is formed an opening 11 b through which pump shaft 3 perforates. Pumpshaft 3 also serves as an input shaft and is disposed along therotational axis of cylinder block 16 and is not relatively rotatablyretained thereto. The upper end of pump shaft 3 projects outwardly fromthe upper wall of upper half housing 1 and fixedly supports an inputpulley 43 having a cooling fan 44. Input pulley 43 is given power from aprime mover (not shown) of the vehicle to which the axle driving systemis mounted through a belt transmission mechanism (also not shown).

[0041] The piston abutting surface of movable swash plate 11 isdesirably slantingly movable from a horizontal state with respect to therotational axis of cylinder block 16, thereby enabling the amount anddirection of discharged oil from the hydraulic pump to be changed. Therear surface of movable swash plate 11 is convex and the inner surfaceof a lid member 15 fixed to upper half housing 1, which closes anopening in the upper wall, is made concave to match with the convex rearsurface of movable swash plate 11. Movable swash plate 11 is constructedto be of a cradle type which, when slantingly moved, slides while cominginto close contact with the concave surface of upper half housing 1.

[0042] In order to slantingly operate movable swash plate 11, as shownin FIGS. 1 and 3, a control shaft 35 extending in parallel to axles 7 isrotatably supported on the right side wall of upper half housing 1opposite to the driving gear train for transmitting power todifferential gear unit 23. A control arm 38 is mounted onto one end ofcontrol shaft 35 outwardly extending from the housing. A swinging arm 39is mounted to the other end of the same, inside the housing. Theswinging arm 39 comprises a first arm 39 a and a second arm 39 b whichextend radially from control shaft 35. A projection 39 c is provided atthe utmost end of second arm 39 b, as shown in FIG. 2. Since controlshaft 35 coincides at the axis thereof with the axis of slanting motionof movable swash plate 11, it is possible to directly engage projection39 c with a groove 11 d formed on a side surface of movable swash plate11. In such a construction, when control arm 38 is rotatedlongitudinally of the vehicle body, swinging arm 39 rotateslongitudinally around control shaft 35 so as to enable movable swashplate 11 to be slantingly moved to thereby change the output of thehydraulic pump.

[0043] At the utmost end of first arm 39 a, opposite to projection 39 c,is disposed an engaging pin 39 d. A bush 51 is fitted onto control shaft35 within the housing. A neutral position return spring 31 of thetorsion coil type is fitted onto bush 51. Both ends of neutral positionreturn spring 31 cross and extend in the direction of first arm 39 a soas to put between both ends an eccentric shaft 33 mounted onto an insidewall of upper half housing 1 and engaging pin 39 a. Accordingly, whencontrol arm 38 and swinging arm 39 rotate to change the speed of thevehicle, one end of neutral position return spring 31 is moved to widena gap between both ends, but the other end of spring 31 is retained bythe eccentric shaft 33, so that control lever 38 is given a biasingforce to return to a neutral position. When the operating force oncontrol arm 38 is released, a restoring force generated at one end ofneutral position return spring 31 holds engaging pin 39 d by eccentricshaft 33 in the specified neutral position. A portion of eccentric shaft33 extending outwardly of the housing is formed into an adjusting screwand eccentric shaft 33 is preferably rotatably shifted therethrough, sothat swinging arm 39 shifts around control shaft 35, thereby enablingmovable swash plate 11 to be adjusted to put it into an accurate neutralposition.

[0044] Control arm 38, as shown in FIG. 2, is provided with an arm 38 bfor connecting a shock absorber 73. A vertical arm 38 a connects to aspeed changing member (not shown), such as a lever or a pedal providedon the vehicle, through a link mechanism (not shown) on the vehicle. Arm38 b is pivotally supported by a movable member of shock absorber 73. Acasing thereof is pivotally mounted onto a support plate 74 fixed to alower surface of an axle housing portion of lower half housing 2. Shockabsorber 73 prevents control arm 38 from abruptly changing speed andalso prevents the speed changing member (not shown) from abruptlyreturning to the neutral position when operating force is released so asto exert a sudden braking action onto the HST. Also, shock absorber 73is positioned somewhat forwardly slanted and extends along the rightside wall of upper half housing 1 straddling axles 7, therebyeffectively utilizing an otherwise unused or dead space surroundingaxles 7.

[0045] Pressurized oil discharged from the hydraulic pump is sent to thehydraulic motor through an oil passage in center section 5. Thehydraulic motor is constructed as shown in FIG. 4. In detail; a cylinderblock 17 is rotatably, slidably mounted on motor mounting surface 41formed on vertical surface 91 of center section 5. A plurality ofpistons 13 are movably mounted in reciprocation in a plurality ofcylinder bores in cylinder block 17, through biasing springs. The headsof pistons 13 abut against a fixed swash plate 37 which is fixedlydisposed between upper half housing 1 and lower half housing 2. Motorshaft 4 is not relatively rotatably retained on the rotational axis ofcylinder block 17 and extends substantially horizontally. One end ofmotor shaft 4 is supported in a bearing bore in motor mounting surface41 of center section 5. The other end is supported by a bearing 76 oninner wall 8 formed along the joint surfaces of upper half housing 1 andlower half housing 2. The utmost end of motor shaft 4 enters into secondchamber R2. Bearing 76 is a sealing bearing for partitioning firstchamber R1 from second chamber R2. An O-ring 77 is disposed between theouter periphery of an outer ring and inner wall 8.

[0046] The driving gear train for transmitting power from motor shaft 4to differential gear unit 23, as shown in FIGS. 1 and 6, comprises agear 25 fixed onto motor shaft 4 where it enters into second chamber R2,a larger diameter gear 24 supported onto a counter shaft 26 andpermanently engageable with gear 25, a smaller diameter gear 21supported on counter shaft 26 and integrally rotatable with largerdiameter gear 24, and ring-gear 22 of differential gear unit 23 which ispermanently engageable with smaller diameter gear 21. Counter shaft 26is disposed in second chamber R2 adjacent to pump shaft 3 andperpendicular thereto. One end of counter shaft 26 is supported by aside wall of the housing at the joint surface of upper half housing 1and lower half housing 2. The other end is supported by inner wall 8 atthe joint surface thereof. The rotational output speed of motor shaft 4is reduced by larger diameter gear 24, smaller diameter gear 21 and ringgear 22 so as to drive axles 7 through differential gear unit 23. Largerdiameter gear 24 on counter shaft 26 is disposed as close as possible tothe outside surface of ring gear 22 and is overlapped axially therewith,thereby reducing the longitudinal length of the housing. In thisembodiment, the HST is disposed to one side of the driving gear train atthe right side thereof. At a further right side thereof is disposed aspeed changing mechanism for the HST. The hydraulic pump thereof ispositioned substantially in the lateral and longitudinal center of thehousing. Differential gear unit 23 is disposed in an enlarged portion ofthe housing.

[0047] A brake disc 19 is fixed on the utmost end of motor shaft 4 insecond chamber R2. As shown in FIGS. 1, 15, 16 and 17, a brake pad 29and a wedge shaped member 70 are interposed between the upper portion ofthe front surface of brake disc 19 and the inner surface of upper halfhousing 1 and are supported thereto, movable only in the direction ofthe rotational axis of motor shaft 4. In a space surrounded by innerwall 8 and the surface of brake disc 19 opposite to brake pad 29 (at theleft side of brake disc 19 in FIG. 15), a biasing member 72 and a brakeoperating shaft 14 are disposed. Brake operating shaft 14 is verticallydisposed and is rotatably supported by upper half housing 1 and lowerhalf housing 2. The upper end of brake operating shaft 14 projectsupwardly from the housing and has a brake arm 27 fixed thereto. On anoutside surface of an intermediate portion of brake operating shaft 14in the housing is formed a flat cutout 14 a which is D-like-shaped whenviewed in cross-section. Arch-like biasing member 72 is fitted intocutout 14 a-and is restricted from axial movement by cutout 14 a and isguided at both sides by the inner surface of upper half housing 1 so asto be slidable only axially of motor shaft 4. Accordingly, when brakearm 27 is rotated to the left or to the right, brake operating shaft 14is rotated. One longitudinal end of cutout 14 a pushes the rear surfaceof biasing member 72 and brake disc 19 is interposed between brake pad29 and biasing member 72 to exert a braking action on motor shaft 4.Wedge member 70 abuts at the lower surface thereof against the upper endof an adjusting bolt 71. Adjusting bolt 71 screws into lower halfhousing 2 and projects outwardly from lower half housing 2, therebyscrewably tightening a lock nut at the intermediate portion of bolt 71for locking wedge member 70. Wedge member 70 is raised or lowered in thehousing as adjusting bolt 71 is rotated so as to advance or retract inthe direction of the rotational axis of motor shaft 4. As brake pad 29is worn, the interval between brake pad 29 and brake disc 19 can beproperly maintained by adjusting bolt 71 which is vertically disposed inlower half housing 2.

[0048] Next, explanation will be given on the construction of centersection 5 in accordance with FIGS. 7 through 14. Center section 5 islarger longitudinally than conventional center sections. Center section5 has three bolt bores 5 h which are open vertically between a frontportion of center section 5 and a rear portion thereof. Center section 5is fixed to upper half housing 1 through bolts. At the center of pumpmounting surface 40 formed on horizontal surface 90 on an upper surfaceof a rear portion of center section 5 is formed a bearing portion so asto enable the lower portion of vertical pump shaft 3 to be rotatablysupported therewith. Pump shaft 3 is perpendicularly disposed withrespect to axles 7. A pair of arcuate ports 40 a and 40 b are open atboth sides of the bearing for suppling and for discharging oil fromcylinder block 16.

[0049] At the front portion of horizontal surface 90 is formed avertical surface 91, a phantom plane which includes vertical surface 91crosses near the longitudinal axis of pump shaft 3. Center section 5 issubstantially L-like-shaped when viewed in cross section. As shown inFIG. 8, a pair of arcuate ports 41 a and 41 b are also vertically openon motor mounting surface 41 formed on front vertical surface 91, sothat oil is adapted to be supplied to or discharged from cylinder block16 through ports 41 a and 41 b. At the center of motor mounting surface41 is provided a bearing for motor shaft 4 which is disposed in parallelto axles 7.

[0050] In order to connect arcuate ports 40 a and 40 b on pump mountingsurface 40 with arcuate ports 41 a and 41 b on motor mounting surface41, a first linear oil passage. 5 a and a second oil passage 5 b arevertically and forwardly bored in a thick portion of center section 5 soas to reduce the lateral length of center section 5.

[0051] Motor mounting surface 41 is positioned in front of thesubstantial center of pump mounting surface 40 so as not to increase thelateral length of the HST when the hydraulic motor is disposed thereon.A third linear oil passage 5 c crosses and communicates with anintermediate portion of second linear oil passage 5 b. Arcuate port 40 aon pump mounting surface 40 is, as shown in FIG. 14, made thinner tocommunicate with first linear oil passage Sa. Arcuate port 40 b is madedeeper to communicate with third linear oil passage 5 c. Arcuate port 41a at the upper portion of motor mounting surface 41 communicates withfirst linear oil passage 5 a. Arcuate port 41 b at the lower portion ofthe same communicates with second linear oil passage 5 b. Second linearoil passage 5 b communicates with third linear oil passage 5 c, wherebyarcuate ports 40 a, 41 a, 40 b and 41 b communicate to form a closedfluid circuit so as to circulate operating oil between the hydraulicpump and the hydraulic motor.

[0052] Check valves 54 and 55 are disposed at the open ends of firstlinear oil passage 5 a and second linear oil passage 5 b and are closedwith lids 64, as shown in FIG. 10. A lid 65 closes the open end of thirdlinear oil passage 5 c. When subjected to pressure, lids 64 and 65 abutagainst projections 2 a and 2 b formed on the inner wall of lower halfhousing 2. A first communication oil passage 5 d is vertically bored incenter section 5 so as to communicate with inlet ports of check valves54 and 55. Oil passage 5 d communicates with a terminal end of a secondcommunication oil passage 5 g which is horizontally bored in centersection 5. A fore end of second communication oil passage 5 gcommunicates with an inlet port 45 a into which discharged oil from acharging pump 45 is guided, as shown in FIG. 12. A plug 66, as shown inFIG. 9, closes the open end of first communication oil passage 5 d.

[0053] Charge pump 45, as shown in FIG. 3, comprises a pump casing whichhas internal teeth for retaining the lower end of pump shaft 3 extendingfrom the horizontal lower surface of center section 5 and external teethengageable with the internal teeth and which is brought into closecontact with the horizontal lower surface of center section 5. Thepump-casing is biased upwardly by a spring interposed between the lowersurface of the pump casing and the inner bottom surface of lower halfhousing 2 and serving also as a relief valve for maintaining a specifiedvalue of pressure of oil discharged from charge pump 45 and filled inthe closed fluid circuit. An annular oil filter 56 is disposed betweenthe inner bottom surface of lower half housing 2 and the horizontallower surface of center section 5 in a manner of surrounding charge pump45, thereby filtering operating oil taken therein.

[0054] As shown in FIGS. 5, 10 and 13, in order to fill the closed fluidcircuit with operating oil after the axle driving system is assembled,oiling pipes 52 and 53 are disposed on the horizontal lower surface ofcenter section 5. Oiling pipe 52 communicates with the deep portion ofarcuate port 41 a through an oil passage vertically bored from thehorizontal lower surface of center section 5. Oiling pipe 53 directlycommunicates with second linear oil passage 5 b. Oiling pipes 52 and 53are exposed at the lower ends thereof from the lower outer surface oflower half housing 2 and are closed by lids after the closed fluidcircuit is filled with operating oil.

[0055] A by-pass operating arm 60, as shown in FIG. 5, is disposed aboveupper half housing 1 so as to open first and second linear oil passages5 a and 5 b into the oil sump for idling axles 7 when hauling thevehicle. In detail, as shown in FIGS. 1 and 4, by-pass operating arm 60is fixed at the base thereof to a by-pass shaft 61 vertically, pivotallysupported to an upper wall of upper half housing 1. Bypass shaft 61extends at the lower end thereof toward the surface of center section 5opposite to motor mounting surface 41 and forms a flat surface at theperiphery of the lower portion.

[0056] A through bore 5 f (see FIG. 8) is open on motor mounting surface41 of center section 5 and is slightly above the center thereof andbetween accurate ports 41 a and 41 b. A push pin 62 is slidablysupported-by center section 5 in the direction of rotation of the axisof cylinder block 17. Push pin 62 can at one end abut against arotatably slidable surface of cylinder block 17 which comes into closecontact with motor mounting surface 41, and abuts at the other endagainst flat surface 61 a of by-pass lever shaft 61.

[0057] When the vehicle is hauled, an operator operates by-passoperating arm 60 outside of the housing causing by-pass shaft 61 torotate. Flat surface 61 a pushes push pin 62 toward cylinder block 17.Push pin 62 releases cylinder block 17 from motor mounting surface 41,and the closed fluid circuit communicates with the oil sump in thehousing through arcuate ports 41 a and 41 b, thereby obtaining freerotation of motor shaft 4.

[0058] Next, explanation will be given on a second embodiment of thepresent invention in accordance with FIGS. 19 through 25, in whichsimilar parts have been given the same reference numerals as used in thedescription of the first embodiment. In the second embodiment, thecenter section is formed in two pieces rather than in one piece as iscenter section 5 in the first embodiment. In this embodiment, centersection 5′ is formed of a first piece 5′a and a second piece 5′b whichare coupled together. On horizontal surface 90 of first piece 5′a isformed pump mounting surface 40. A pair of kidney-shaped ports 40 a and40 b are open on pump mounting surface 40. On a side surface of avertical portion 91 of second piece 5′b is formed motor mounting surface40, on which a pair of kidney-shaped bores 41 a and 41 b are open.Communicating oil passages 100 and 101 are bored in first piece 5′a. Theterminal ends thereof are open on the side surface. Inside second piece5′b are bored oil passages 102 and 103 which communicate with the pairof kidney-shaped ports 41 a and 41 b. The terminal ends of the passages102 and 103 are open on the side surface. Oil passages 100 and 102, 101and 103 connect with each other through the joint surfaces whenhorizontal portion 90 is coupled with vertical portion 91, therebyforming a closed fluid circuit for circulating therein operating oilbetween the hydraulic pump and hydraulic motor.

[0059] Center section 5′ is not provided with bolt insertion bores 5 has shown in the first embodiment, but is sandwiched between upper halfhousing 1 and lower half housing 2 so as to be restrained from verticaland lateral movements, thereby being fixedly positioned in the housing.

[0060] The advantages of a two-piece center section 5′ include that themanufacturing and processing costs and the number of assembly processesare reduced, which reduces the overall cost of the system. Further,fewer parts are required in that bolts for securing the center sectionin the housing are not required.

[0061] When oil leaks caused from the closed fluid circuit in centersection 5′, oil in first chamber R1 is taken into the closed fluidcircuit through oil filter 56 and check valves (not shown). In thisembodiment, control shaft 35 for slantingly rotating swash plate 11 ofthe hydraulic pump is vertically and rotatably supported by an upperwall of upper half housing 1. Such construction for engaging controlshaft 35 with swash plate 11 is the same as, for example, that describedin U.S. Pat. No. 5,495,712 which is incorporated herein by referencethereto in its entirety.

[0062] As seen from the above description, the axle driving system ofthe present invention can be applied to drive the axles of a vehicle soas to sufficiently reduce the mounting space thereof. Vehicles on whichthis axle driving system may be used include agricultural workingvehicles such as lawn and garden tractors, and transportation vehicles.

[0063] Although several embodiments have been shown and described, theyare merely exemplary of the invention and are not to be constructed aslimiting the scope of the invention which is defined by the appendedclaims.

What is claimed is:
 1. An axle driving system, comprising: a housing; ahydrostatic transmission disposed in said housing including an inputmeans, a hydraulic pump driven by said input means, a hydraulic motordriven by pressurized oil from said hydraulic pump, and an output meansdriven by said hydraulic motor; an axle, wherein said hydraulic pump isdisposed between said hydraulic motor and said axle; and a driving geartrain disposed in said housing for drivingly connecting said outputmeans of said hydrostatic transmission and said axle.
 2. An axle drivingsystem according to claim 1, further comprising: a pump mounting surfacefor mounting said hydraulic pump provided in said housing adjacent tosaid axle; and a motor mounting surface for mounting said hydraulicmotor provided in said housing away from said axle; wherein saidhydraulic pump is mounted substantially vertically on said pump mountingsurface, and said hydraulic motor is mounted substantially horizontallyon said motor mounting surface.
 3. An axle driving system according toclaim 2, wherein a phantom plane which includes said motor mountingsurface intersects the rotational axis of said hydraulic pump.
 4. Anaxle driving system, comprising: a housing; a hydrostatic transmissiondisposed in said housing including an input means, a hydraulic pumpdriven by said input means, a hydraulic motor driven by pressurized oilfrom said hydraulic pump, and an output means driven by said hydraulicmotor; an axle, wherein said input means is positioned at thesubstantial center of the lateral width of said housing, extendsvertically with respect to said axle and receives power from a primemover; and a driving gear train disposed in said housing for drivinglyconnecting said output means of said hydrostatic transmission-and-saidaxle.
 5. An axle driving system according to claim 4, wherein said inputmeans is positioned at the substantial longitudinal center of saidhousing.
 6. An axle driving system, comprising: a housing; a hydrostatictransmission disposed in said housing including an input means, ahydraulic pump driven by said input means, a hydraulic motor driven bypressurized oil from said hydraulic pump, and an output means driven bysaid hydraulic motor; an axle, wherein said input means is positioned atthe substantial longitudinal center of said housing, extends verticallywith respect to said axle and receives power from a prime mover; and adriving gear train disposed in said housing for drivingly connectingsaid output means of said hydrostatic transmission and said axle.
 7. Anaxle driving system according to claim 6, wherein said input means ispositioned at the substantial center of the lateral width of saidhousing.
 8. An axle driving system, comprising: a housing, wherein saidhousing has a first section and a second section; a hydrostatictransmission disposed in said first section of said housing including aninput means, a hydraulic pump driven by said input means, a hydraulicmotor driven by pressurized oil from said hydraulic pump, and an outputmeans driven by said hydraulic motor; an axle, wherein said hydraulicpump is disposed between said hydraulic motor and said axle in saidfirst section of said housing; and a driving gear train disposed in saidsecond section of said housing for drivingly connecting said outputmeans of said hydrostatic transmission and said axle.
 9. An axle drivingsystem according to claim 8, wherein said housing further comprises: aninner wall partitioning said first section of said housing from saidsecond section of said housing.
 10. An axle driving system according toclaim 9, wherein said housing further comprises: two housing membersjointed on a plane substantially parallel to said axle, wherein aplurality of inner wall portions are provided on said housing membersand extend at one end toward said plane so as to overlap with each otherwhen said housing members are joined, thereby forming said inner wall.11. An axle driving system according to claim 8, wherein said firstsection of said housing and said second section of said housing areindependent of each other.
 12. An axle driving system according to claim11, further comprising: an inner wall provided in said housingpartitioning said first section of said housing from said second sectionof said housing; a bore provided in said inner wall, wherein said outputmeans faces said second section through said bore and is connected tosaid driving gear train through said bore; and an oil seal meansdisposed on an inner peripheral surface of said bore.
 13. An axledriving system according to claim 12, further comprising: an oilflow-through bore provided in said inner wall; and an oil filtercovering said oil flow-through bore; wherein oil stored in said firstsection of said housing and said second section of said housing freelycirculates through said oil filter.
 14. An axle driving system accordingto claim 11, further comprising: an external chamber for adjusting thevolume of oil used by said hydrostatic transmission stored in said firstsection, wherein said external chamber is fluidly connected to one ofsaid first section and said second section of said housing.
 15. An axledriving system, comprising: a housing; a hydrostatic transmissiondisposed in said housing including an input means, a hydraulic pumpdriven by said input means, a hydraulic motor driven by pressurized oilfrom said hydraulic pump, and an output means driven by said hydraulicmotor; an axle, wherein said hydraulic pump is disposed between saidaxle and said hydraulic motor; and a driving gear train disposed in saidhousing including an intermediate rotation transmitting member fordrivingly connecting said output means of said hydrostatic transmissionand said axle, wherein said hydraulic pump is disposed adjacent to saidintermediate rotation transmitting member.
 16. An axle driving systemaccording to claim 15, wherein a rotational axis of said intermediaterotation transmitting member-extends substantially perpendicular to arotational axis of said hydraulic pump.
 17. An axle driving systemaccording to claim 15, wherein a rotational axis of said hydraulic pumpand a rotational axis of said output means are substantiallyperpendicular to each other, the rotational axis of said output meansextends substantially parallel to a rotational axis of said axle, and arotational axis of said intermediate transmitting member extendssubstantially parallel to the rotational axis of said axle.
 18. An axledriving system according to claim 17, further comprising: an output gearprovided on-said output means; a first gear and a second gear disposedon said intermediate rotation transmitting member; and a third gearprovided on said axle; wherein said first gear engages with said outputgear and said second gear engages with said third gear, thereby formingsaid driving gear train.
 19. An axle driving system according to claim18, wherein said second gear is disposed between said first gear andsaid hydraulic pump.
 20. An axle driving system, comprising: a housing;a hydrostatic transmission disposed in said housing including an inputmeans, a hydraulic pump driven by said input means, a hydraulic motordriven by pressurized oil from said hydraulic pump, an output meansdriven by said hydraulic motor, and a center section on which saidhydraulic pump and said hydraulic motor are mounted and are fluidlyconnected with each other; an axle; and a driving gear train disposed insaid housing for drivingly connecting said output means of saidhydrostatic transmission and said axle; wherein said center section isseparably mounted in said housing, so that a first portion is providedadjacent to said axle and a second portion is provided away from saidaxle, a pump mounting surface for mounting said hydraulic pump isprovided on said first portion, and a motor mounting surface formounting said hydraulic motor is provided on said second portion.
 21. Anaxle driving system according to claim 20, wherein said pump mountingsurface and said motor mounting surface are perpendicular to each other,the rotational axis of said hydraulic pump disposed on said pumpmounting surface is substantially vertical, the rotational axis of saidhydraulic motor disposed on said motor mounting surface is substantiallyhorizontal, and said first portion and said second portion of saidcenter section are coupled to each other so that the phantom plane whichincludes said motor mounting surface intersects said first portion. 22.An axle driving system according to claim 21, wherein the phantom planewhich includes said motor mounting surface intersects a rotational axisof said hydraulic pump disposed on said pump mounting surface.
 23. Anaxle driving system according to claim 21, wherein said center sectionfurther comprises: a pair of first kidney-shaped ports open on said pumpmounting surface for receiving oil supplied or discharged into or fromsaid hydraulic pump; a pair of second kidney-shaped ports open on saidmotor mounting surface for receiving oil supplied-or discharged into orfrom said hydraulic motor; and a pair of oil passages provided in saidcenter section for fluidly connecting said first kidney-shaped ports andsaid second kidney-shaped ports; wherein said oil passages arepositioned vertically in a thick portion of said center section.
 24. Anaxle driving system, comprising: a housing; a hydrostatic transmissiondisposed in said housing including an input means, a hydraulic pumpdriven by said input means, a hydraulic motor driven by pressurized oilfrom said hydraulic pump, and an output means driven by said hydraulicmotor; a speed changing mechanism for changing the rotational directionand rotational speed of said output means, wherein said speed changingmechanism is disposed at one side of said hydrostatic transmission; anaxle, wherein said hydraulic pump and said hydraulic motor are disposedsubstantially perpendicular to said axle; and a driving gear traindisposed in said housing for drivingly connecting said output means ofsaid hydrostatic transmission and said axle.
 25. An axle driving systemaccording to claim 24, wherein said driving gear train is disposed at aside of said hydrostatic transmission opposite to said speed changingmechanism, so that said hydrostatic transmission is disposed betweensaid speed changing mechanism and said driving gear train.
 26. An axledriving system according to claim 24, wherein said hydraulic pump is ofvariable displacement type and further comprises: a movable swash plate;a speed change shaft provided on said speed changing mechanism, whereinthe rotational axis of said speed change shaft extends substantiallyparallel to said axle and interlocks with said movable swash plate; anda neutral position return bias means provided on said speed changingmechanism for returning said movable swash plate to a neutral position.27. An axle driving system according to claim 25, wherein said speedchanging mechanism further comprises: a control arm provided at one sideof said housing and mounted onto said speed change shaft, wherein saidcontrol arm is swingable around the horizontal rotational axis thereof;and a shock absorber disposed on one side surface of said housing andmounted at one end to said control arm and at an other end to saidhousing.
 28. An axle driving system, comprising: a housing; ahydrostatic transmission disposed in said housing including an inputmeans, a hydraulic pump driven by said input means, a hydraulic motordriven by pressurized oil from said hydraulic pump, and an output meansdriven by said hydraulic motor; a speed changing mechanism for changingthe rotational direction and rotational speed of said output means; anaxle, wherein said hydraulic pump and said hydraulic motor are disposedsubstantially perpendicular to said axle; a driving gear train disposedin said housing for drivingly connecting said output means of saidhydrostatic transmission and said axle; and a brake unit providedbetween said output means and said axle, wherein said brake unit isdisposed at one side of said hydrostatic transmission.
 29. An axledriving system according to claim 28, wherein said speed changingmechanism is disposed at the side of said hydrostatic transmissionopposite to said brake unit so that said hydrostatic transmission isdisposed between said brake unit and said speed changing mechanism. 30.An axle driving system according to claim 28, wherein said brake unit isdisposed in said housing.
 31. An axle driving system according to claim28, further comprising: a brake operating shaft provided at said brakeunit, wherein said brake unit is actuated by rotating said brakeoperating shaft around the longitudinal rotational axis thereof; whereinsaid input means extends substantially perpendicular to said axle andthe longitudinal rotational axis of said brake operating shaft extendssubstantially parallel to the rotational axis of said input means. 32.An axle driving system according to claim 31, wherein said brakeoperating shaft is rotatably supported by the upper wall of said housingin a substantially vertical position and an operating end of said brakeoperating shaft is disposed in said housing.
 33. An axle driving systemaccording to claim 32, wherein said brake unit further comprises: abrake disc provided at said brake unit and integrally rotatable withsaid output means, wherein a rotational axis of said brakedisc-intersects substantially perpendicularly with a longitudinal axisof said brake operating shaft; a first friction member and a secondfriction member disposed opposite to a rotary surface of said brakedisc, wherein said first friction member and said second friction memberare supported within said housing and said first friction member ismovable toward and away from the rotary surface of said brake disc; afirst cam provided on said brake operating shaft; and a second camprovided at a rear surface of said first friction member and engageablewith said first cam; wherein said first cam and said second cam convertrotational motion of said brake operating shaft into linear motion ofsaid first friction member, thereby pressing said first friction memberonto said brake disc.
 34. An axle driving system according to claim 33,wherein said first cam and said second cam are formed on a flat surfacesubstantially parallel to said rotary surface of said brake disc.
 35. Anaxle driving system according to claim 33, wherein said second frictionmember is adjustable from the exterior of said housing toward and awayfrom said rotary surface of said brake disc.